1. Field of the Invention
The present invention relates to a wide-angle zoom lens system which is used for a digital camera, which has a half angle-of-view exceeding 35° at the short focal length extremity, and has a zoom ratio of approximately 5.
2. Description of the Prior Art
According to miniaturization of pixels in a imaging-forming device (e.g., pixels of a CCD), there has been a demand for higher resolution in a photographing lens system which is arranged to form an image on an image-forming surface of the CCD.
Moreover, a space is required to position a filter group in front of the CCD, so that a longer back focal distance is required. Still further, in order to prevent shading or color shift, telecentricity, i.e., leading the exit light rays from the final surface (the most image-side surface of the photographing lens system) onto the image-forming surface of the CCD so that the exit light rays is incident on the image-forming surface of the CCD at a right angle.
In a wide-angle zoom lens system of the prior art, a negative-lead type zoom lens system (a negative lens group being the most object-side lens group) is often employed for up to a zoom ratio of approximately 4. However, a negative-lead type zoom lens system cannot attain a large zoom ratio. Therefore a positive-lead type zoom lens system (a positive lens group being the most object-side lens group) is employed to attain a zoom ratio 5 or more.
As examples of the prior art, Japanese Unexamined Patent Publication No. H05-027167 discloses a positive-lead type wide-angle zoom lens system which has a zoom ratio of approximately 7 to 8. However, the half angle-of-view at the short focal length extremity is only about 25°. Consequently, it is difficult to make the focal length at the short focal length extremity shorter.
Japanese Unexamined Patent Publication No. H07-020381 discloses a zoom lens system having a half angle-of-view of about 38°, which means that the focal length at the short focal length extremity can be made shorter. However, the diameter of the most object-side lens group is large, and a large number of lens elements are required, so that miniaturization thereof is insufficient.